1. Field of the Invention
This invention relates to a method and apparatus for mixing liquid within a vessel employing a mixing system utilizing both pump-generated flow and induced flow to thoroughly mix the contents of the vessel. More particularly, it is concerned with a method and apparatus for mixing liquid material having suspended particulates which have a tendency to settle and collect on the bottom of the container or vessel such as a sewage digester, whereby the liquids may be continuously mixed to produce an enhanced distribution of the various layers and settleable particulates throughout the contents of the vessel. This mixing is accomplished by the use of at least one pump oriented to direct the flow generated thereby into a draft tube having a larger diameter than the outlet diameter of the pump in order to produce an induced flow of liquid through the draft tube.
2. Description of the Prior Art
It has heretofore been well known to mix liquids of varying specific gravities or having suspended solids therethrough in an attempt to achieve thorough and uniform distribution of the liquids throughout the vessel. Mixing may be important in a variety of contexts, including chemical refining, food manufacturing, and waste water treatment. In some of these industries, liquids may contain suspended particulate matter which must constantly flow to avoid creation of a stratified condition of various components of the mixture, or to avoid settlement of the particulate matter.
In larger containing vessels, the tendency of certain particulate materials to settle may produce a compacted layer along the bottom of the vessel. For example, in sewage treatment and waste water processing, suspended particulate matter may be sequentially transferred to a series of vessels until reaching a final containment vessel. The liquid transferred into this final vessel may contain a high proportion of suspended solids which, if not continuously agitated, will settle to produce a hardened layer at the bottom of the vessel which is resistant to fluid transfer and may be removed only be dredging, digging or the like. Similarly, in petroleum tanks holding crude oil, heavy components such as asphalt and tar may tend to settle prematurely unless agitated and continuously mixed.
In sewage treatment systems, such as anaerobic digesters, it is especially important to achieve thorough mixing of the contents of the vessel in order to achieve not only a uniform distribution of the digestible materials within the liquids, but also to strive for near uniform temperature distribution within the vessel. It has been found that anaerobic digestion is most effective when the temperature of the liquid is within a few degrees of about 95 degrees Fahrenheit, and thus the most efficient digestion of the liquid is achieved when the temperature is nearly constant throughout the vessel in this range.
Another problem encountered in such digestors is the accumulation of a layer of scum at the surface of the vessel. Such scum resists mixing and builds up in a gradually thickening layer until the efficiency of the digestion process within the vessel is reduced.
Thus, it is important to alleviate the accumulation of scum and thoroughly integrate the liquid components of the vessel into a nearly homogenous mixture. It has been found that by continuously mixing the scum which accumulates at the top of the vessel into the remaining liquids, the complete digestion of the liquid will be quickly and efficiently accomplished. In ordinary digestion processes, the liquid contents of the vessel tend to segregate into layers of a froth or scum at the top, a liquid layer in an intermediate region, and a layer of sediment or sludge at the bottom. Yet further, in anaerobic digestion, methane gas is generated as a useful by-product of the anaerobic digestion process, which accumulates above the layer of froth or scum and is trapped beneath a cover on the vessel. Ordinarily, the methane will be evacuated for use as a fuel, sometimes known as "biogas", while the sediment or sludge is pumped or drained from the bottom of the tank for use agriculturally as a fertilizer or otherwise disposed of. The liquid may be drawn off for further treatment and processing, while the scum layer must be dealt with as a waste by-product of the sewage treatment. However, when the material within the tank has been thoroughly and completely, digested, such scum layer is often kept to a minimum and presents few difficulties in disposal.
A number of various processes for achieving a thorough distribution of sediment or sludge have heretofore been known. The problems of thorough mixing having been previously encountered and others have attempted to achieve mixing of the liquids within a vessel through a draft tube generally oriented at the center of the vessel. In one approach, a pump, agitator or propellor is located within the draft tube in order to circulate the liquid therethrough, as illustrated for instance by U.S. Pat. Nos. 2,024,986, 2,048,640, 2,067,161, 2,076,529, 2,337,507, 2,359,004, 2,597,802, 2,678,915, 2,788,127, 2,875,151, 3,194,756, 3,470,092, 3,827,679, 4,045,336, 4,188,289, 4,192,740, 4,246,111, 4,290,885, 4,330,407, 4,388,186, 4,394,268, 4,613,434, and 4,746,433.
In another approach, movement of the liquid through the draft tube has been achieved by gas lift, whereby a source of compressed gas is allowed to expand and rise through the draft tube, carrying with it a quantity of liquid, as shown for example in U.S. Pat. Nos. 3,152,982, 3,682,313, 3,910,838, 4,039,439 and 4,793,929.
Yet further, some have sought to achieve thorough intermixing by a combination of these approaches, as shown in U.S. Pat. Nos. 2,786,025, 3,092,678, 3,724,667, 3,953,326, 4,076,515, and 4,242,199.
However, such approaches have not generally utilized a combination of primary flow and induced flow of the accumulated liquid to achieve maximum efficiency of the mixing process, but also have failed to successfully address the problem of the extremely thick and difficult to pump materials which may accumulate in a final settling vessel or digester which must be continuously agitated to avoid solidification. Finally, the problems presented by the build up of froth or scum at the upper layer of the liquid should preferably also be addressed by a simple, easy to construct and maintain apparatus.